The present invention relates to a method for separating and purifying a desired crystalline substance from a solid mixture and for removing occluded impurities by repeated dissolution and recrystallization. It also relates to apparatus suitable for use in practicing the method.
Because of the structure of the lattice cell, crystallization theoretically offers the advantage over other separation processes of yielding a pure component in a single stage. In practice, however, it is generally not possible to obtain a pure component by a single crystallization because of occlusions of mother liquor. These occlusions, or small pockets of solution trapped within or between relatively large individual crystals, can be removed by redissolving the impure crystals in pure solvent. The pure solvent dilutes impurities from the occluded solution and, upon subsequent recrystallization, a higher degree of purity is achieved. Consequently, multiple crystallizations can provide any desired degree of purity, limited only by the purity of the solvent.
Most production crystallizers for commodity chemicals are large, expensive, automated devices which achieve supersaturation in continuous operation, either by cooling of hot concentrated solutions or by evaporation of solvent within the crystallizer. Prolonged suspension of growing crystals in agitated mother liquor effectively minimizes the number and extent of occlusions and provides some measure of particle size control. Attempts to stockpile crystal and mother liquor inventories of intermediate purity to achieve multistage countercurrent crystallization with a single unit become increasingly unattractive as the material value and the number of required crystallizations increase because of scheduling complexity, storage requirements, and carrying costs.
Both the evaporative and the cooling type commercial crystallizers require a substantial investment in energy in order to recover any crystals, and this cost penalty is multiplied by whatever number of recrystallizations are required for achieving the desired level of purity. Furthermore, addition of more water to redissolve purified crystals for succeeding crystallization steps multiplies the water purification cost and/or the contribution of soluble impurities in this water to the ultrapure crystal.
Daily feed batch and mother liquor sampling and very prompt analyses of these samples for impurities are generally required for control of product purity levels during operation of a continuous crystallizer. The results of the various analyses are considered during calculation of the minimum volume of mother liquor which must be purged from the crystallizer that day to ensure that the product will meet purity specifications during the next 24 hours. Larger purges result in a cleaner product, but at the cost of reduced material efficiency and/or the need to reprocess a greater amount of impure mother liquor material. Mother liquor purging also frequently results in large upsets in the crystal size distribution, which may lead to rejection of some product for reasons unrelated to its level of purity.
The demand for crystalline commodity chemicals is usually met with continuous crystallizers because of their low labor input and predictable behavior, but the purity level of the product is geared by simple economics toward the purity specifications of 90% of the market. Customers who may require more modest amounts of material with impurity concentrations which are lower by several orders of magnitude are usually forced to repurify the commodity material themselves or obtain it from a specialty manufacturer of fine chemicals. Continuous crystallizers are almost never used in these situations because of their prohibitive cost and the intermittent nature of the market. For these reasons the cost per pound of purified crystalline materials typically climbs very steeply as purity specifications are tightened and potential demand for higher purity grades is curtailed accordingly.
It is an object of the invention to provide a countercurrent multistage crystallization process for producing ultrapure crystalline compounds wherein dissolution and crystallization occur within each of a series of modular devices.
Another object of the invention is to provide inexpensive modules suitable for performing the process of the invention and which can be combined in series to achieve any desired degree of purity with only a modest increase in the amount of labor required.
Other objects and advantages of the invention will be apparent from the following description, drawings, and disclosure.